The shafts of such transmissions form a driving shaft and a driven shaft. One disk of each pair is axially displaceable and part of a hydraulic gripping or clamping mechanism, acted upon by pressurized fluid and firmly joined to the associated shaft, for generating the compression forces upon a traction element running between the cone disks. For setting and maintaining the transmission ratio, the gripping mechanism is controlled by pressurized fluid and a control valve, typically a spool valve, which is settable by a control element which controls pressure fluid in lines coupled to the control valve. Torque sensors located on the transmission shafts in the flow of torque are located in a pressure fluid return line coupled to the spool valve; by a torque-dependent relative motion of two valve parts relative to one another, they throttle the system pressure as a function of load and thus determine the pressure fluid pressure on the gripping or clamping mechanism, controlling the pressure as a function of load. The torque sensors are controllable in their operation as a function of the operating state of the transmission.
Cone disk transmissions of this kind are known for instance from U.S. Pat. No. 4,261,213 and earlier German patent 18 16 950. Good disk gripping conditions are obtained if, as taught particularly by the U.S. patent, a torque sensor disposed on the drive mechanism determines the basic pressure of the hydraulic system, and this basic pressure acts upon the driven side of the transmission. With the aid of the control valve, a necessarily higher pressure is mainted on the drive side as known in cone disk transmissions to obtain the desired transmission ratio.
The torque sensor on the driving side is hydraulically inserted into the return line of the control valve; FIG. 10 of the aforementioned U.S. Pat. No. 4,261,213 shows such a system. In that construction, a hydraulic changeover switch, i.e. a valve, is provided which, upon an operationally dictated change between the driving shaft and the driven shaft, always puts the torque sensor on the applicable drive mechanism into action. In terms of its switching position, the changeover switch is acted upon by the respective pressures of the fluid on the driving and driven sides, and its switching position is determined by the higher pressure fluid pressure on the applicable drive side.
This known system function only with considerable inertia, however. A change in load direction between the two sides, interchange of driving and driven sides of the transmission requires that the control valve must first react to the imbalance in forces between the two transmission sides that is, driving and driven sides, before a changeover can be made. It is accordingly not well suited for applications where the gripping system must adjust rapidly to the change in load direction, i.e. direction of power flow.
The aforementioned disadvantage arises particularly when the cone disk transmission is used along with a planetary gear in a multirange transmission of the kind described in German patent Disclosure Document DE-OS 29 48 681. The changeover from one gear range to another with the aid of clutches causes a sudden change in load direction for the cone disk transmission.